Materials characterization cell for polarization spectrum and streaming electrification measurements

ABSTRACT

A materials characterization test cell is disclosed. The test cell includes a housing including spacers for supporting sample insulation materials, and a channel disposed in the housing to permit the passage of oil through the housing. A pair of electrodes is provided to provide a mechanism for making polarization spectrum and/or streaming electrification measurements.

This is a division of application Ser. No. 08/484,571, filed Jun. 7,1995, the disclosure of which is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to the field of powertransformers and insulation thereof, and more particularly to amaterials characterization cell for measuring polarization spectra andstreaming electrification characteristics of insulation materials andcombinations of insulation materials and dielectric fluids.

BACKGROUND OF THE INVENTION

Polarization spectrum measurements provide information about the stateof transformer insulation. Such measurements have the distinct advantageof being non-invasive and are thus ideal for field applications. Thepolarization spectrum is known to be dependent upon both aging andmoisture content, which are of great interest to power transformermanufacturers. However, calibrating and understanding the dependenciesof the polarization spectrum on the age and moisture content ofinsulation can be extremely difficult. One object of the presentinvention is to provide a materials characterization test cell thatcombines oil and cellulose insulation in a well-defined geometry, whichtest cell is useful in resolving the difficulty in calibrating andunderstanding said dependencies. The materials calibration test cell canbe integrated into an external system to support oil flow and can alsobe used to examine streaming electrification.

Streaming electrification has been identified as a hazard for largeforced-oil-cooled power transformers. This phenomenon is traditionallyquantified by measuring the electrostatic charging tendency (ECT) of thetransformer oil. However, this measurement is conventionally made bypassing the oil through a filter and does not address the physicalprocess occurring in a transformer. Another objective of the presentinvention is to provide a materials characterization test cell andmethod of use thereof for determining the charging tendency of aliquid/solid insulation combination. The invention employs the noveltest cell, which allows a dielectric liquid to flow against solidinsulation samples in a well-defined geometry. The charging tendenciesof various transformer insulation components can be easily determinedwith the present invention.

Polarization Spectrum

The polarization spectrum as a function of the energizing frequencyencompasses a wide range of physical phenomena. At very high (optical)frequencies, electron behavior is evident while at very low (DC)frequencies, space and surface charge effects dominate. For transformerdiagnostics, the latter case is of interest since the spectrum canprovide information about the state of the insulation in thetransformer. The degree of insulation aging and moisture content bothsignificantly influence the polarization spectrum, and therefore thespectrum is indicative of the age and moisture content of theinsulation.

A dielectric material will be polarized by the application of anelectric field. One method of characterizing polarization is through therecovery voltage method (RVM). The geometric capacitance of theinsulation causes the build-up of a charge which is discharged after thesample is energized with a DC voltage for a specified period of time.The voltage appearing across a dielectric is measured once the shortcircuit is opened. The spectrum is obtained by sweeping the energizingand short circuit times over the range of interest. The peak recoveryvoltage is measured over this range, and the recovery voltagecharacteristic typically exhibits a peak whose location and magnitudevary with the insulation properties. Typical results for transformerpolarization spectra show that cellulose aging and/or moisture contentcan shift the recovery voltage peak.

A polarization spectrum analyzer (e.g., one made by TETTEX) can bereadily attached to the bushing of a transformer. This non-invasiveanalyzer provides only a macroscopic characteristic of a very large andcomplex insulation structure. The results are thus difficult to explainand understand in terms of the microscopic physical phenomena ofinterest. The materials characterization test cell provided by thepresent invention provides an ideal mechanism for microscopicallystudying the very large and complex insulation structure employed by atransformer. The invention provides a well-defined oil/cellulose systemthat can be examined under very controlled conditions with both staticand flowing oil. The cell geometry allows for different solid samplesover a range of thickness to be easily installed and tested.

Streaming Electrification

Despite strict industrial specifications, transformer oils may containtrace amounts of impurities. When such impurities are dissociable innature, positive and negative ions may be formed. If the oil is placedin contact with a solid material, one ionic species will bepreferentially adsorbed. As a result of such adsorption, an excess ofthe other species will remain in the oil. Under static conditions, theseexcess charges occupy a Boltzmann distribution near their counterpartsat the liquid/solid interface. This double layer at the interface isestablished in accordance with the static relaxation equation such thatcharge is conserved.

When the oil is circulated for cooling purposes, the entrained ions areconvected away from the counterions adsorbed in the solid. Staticpotentials may be generated, which can compromise the dielectricintegrity of the transformer. This phenomenon is traditionallyquantified in terms of the oil. The ECT can be determined by passing theoil through a filter and measuring the resulting streaming current. TheECT for the oil can then be calculated based upon the flow rate. Thedegree of streaming electrification for various oils can be compared inthis manner.

The disadvantage of the known method for performing ECT measurements isthat the role of the solid phase is not adequately addressed. Inreality, the nature of the cellulose surface may significantly influencethe electrification process. Furthermore, the flow conditions existingin a filter are much different from those in a transformer duct. Thepresent invention provides a mechanism for obtaining charging tendencymeasurements of a liquid/solid dielectric system, and the geometryallows solid samples to be easily installed for testing against varioustransformer oils in a plane channel geometry. The invention provides amuch more realistic assessment of streaming electrification in powertransformer insulation.

SUMMARY OF THE INVENTION

A materials characterization cell in accordance with the presentinvention comprises a housing, a channel disposed in the housing topermit the static or dynamic passage of oil through the housing, a firstsample of insulation material disposed in the housing adjacent to thechannel, and a first electrode disposed in the housing adjacent to thefirst sample. The presently preferred embodiment of the invention alsoincludes a second sample of the insulation material disposed in thehousing adjacent to the channel and opposite the first sample, and asecond electrode disposed in the housing adjacent to the second sample.This embodiment provides a capacitor having a prescribed geometry, whichis formed by the electrodes, samples, and channel.

The present invention also provides a materials characterization systemincluding a materials characterization cell as described above, and apolarization spectrum analyzer coupled to the materials characterizationcell. The presently preferred embodiment of the system also includes ameans for providing a controlled flow of oil through the materialscharacterization cell.

An alternative preferred embodiment of the present invention comprisesan electrometer instead of or in addition to the polarization spectrumanalyzer. This embodiment is employed to measure the charging tendencyof a combination of liquid and solid insulation materials. In oneexemplary embodiment of the present invention, the electrometer isaccurate to within 0.1×10⁻¹² A and has a settling time of approximately1 second. A Keithbly model 617 or similar electrometer may be used tocarry out the present invention.

Other features and advantages of the present invention are disclosedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an end view of a materials characterization cell inaccordance with the present invention.

FIG. 2 depicts a side view of the materials characterization cell ofFIG. 1.

FIG. 3 schematically depicts a materials characterization system inaccordance with the present invention.

FIG. 4 is a plot of recovery voltage over time (i.e., the polarizationspectrum) for various material types and thicknesses, as determined by apolarization spectrum analyzer. The data depicted in this plot wereobtained with a materials characterization test cell in accordance withthe present invention.

FIG. 5 is a plot of the ECT of various materials as determined with thematerials characterization cell of the present invention. This plotshows that streaming electrification is a function of the solidinsulation component in addition to the liquid, which in this case wasExxon Univolt Transformer Oil.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One presently preferred embodiment of the materials characterizationcell in accordance with the present invention is depicted in FIGS. 1 and2. As shown, the inventive materials characterization cell 10 comprisesa LEXAN housing 12 including spacer portions 14, cellulose or pressboard samples 16, a channel 18, and aluminum electrodes 20. The LEXANspacers 14 support the samples 16 such that the geometry of the channel18 is independent of the sample thickness. As indicated by the arrow inFIG. 2, the channel 18 allows for the passage of oil 22 through the cell10. Oil can flow through the cell in either direction, i.e., in thedirection of the arrow or in the opposite direction.

FIG. 3 depicts one presently preferred embodiment of a materialscharacterization system in accordance with the present invention. Asshown, this system employs the test cell 10; a pair of mounting flanges30a, 30b; a pair of valves 32a, 32b; a pair of aluminum tanks 34a, 34b;depth gauges or indicators 36a, 36b for the respective tanks; a secondpair of valves 38a, 38b; a pair of vent ports 40a, 40b associated withthe valves 38a, 38b; and inlet/outlet ports 42a, 42b. The two identicaltanks 34a, 34b are employed as oil reservoirs. The flange arrangement isprovided to facilitate installation of the cell 10. Oil is forcedthrough the cell by pressurizing one tank with dry nitrogen and ventingthe other to atmosphere. The depth gauges 36a, 36b provide a mechanismfor measuring the volume of oil dispensed through the cell 10, which istimed such that the flow rate can be estimated. Typical oil velocitiesare on the order of 1 meter per second and the flow is laminar with aReynolds number of approximately 200. The dimensions of the materialscharacterization cell 10 provide for fully developed flow over most ofthe length of the oil channel 18. A polarization spectrum analyzer (orsuitable electrometer for streaming electrification measurements, asdiscussed below) can be connected across the cell electrodes 20.

In light of the long times required for polarization spectrummeasurements, a pump may be added to sustain a steady flow of oil overthe course of the experiment. The pump may be installed in series withthe reservoir tanks to complete a hydraulic circuit. Any chargesgenerated due to the streaming electrification in the pump will thus beallowed to relax in the tanks before the oil enters the cell 10. In thismanner, the influence of flow dynamics upon the polarization spectrummay be investigated as a function of insulation material and thickness.

FIG. 4 depicts polarization spectra obtained with the system of FIG. 3,for aluminum electrodes, plexiglass, 0.03 mil capaco, 0.005 mil drypaper, and 0.031 mil EHV Hi-Val insulation materials. All of thesematerials, except for bare aluminum, are dielectrics. The listeddielectrics are types of insulation used in transformers. FIG. 4demonstrates the polarization spectrum associated with each material (inoil). This data can be used to evaluate the state of insulation overtime.

The materials characterization cell 10 described above employselectrodes 20 that are fitted with O rings to provide a seal and aresecured with DELRIN compression bolts. As discussed above, the celldimensions are selected to provide for fully developed flow over most ofthe channel length. Streaming current measurements can be obtained fromthe aluminum electrodes 20 using a suitable electrometer. The cell ispreferably shielded with a Faraday cage such that currents in thepicoampere range can be measured reliably. Based on the calculated flowrate Q, the charging tendency of a given oil/cellulose combination isdetermined in terms of the streaming current I as q=I/Q. The chargingtendency can be evaluated in terms of the solid sample material,thickness, and surface characteristics for various transformer oils orother dielectric liquids. The solid samples may be supplied in sheetform and machined to fit into the cell.

FIG. 5 depicts the results of charging tendency measurements obtainedwith the system depicted in FIG. 3. The results show that streamingelectrification is clearly a function of the solid insulation componentand the liquid.

It should be noted that the scope of protection following claims is notlimited to the presently preferred embodiments described above. Forexample, the cell could be made a different size. A larger size with ahigher flow rate would yield higher signals for measurement.

We claim:
 1. A materials characterization system, comprising:(a) amaterials characterization cell; (b) a polarization spectrum analyzercoupled to said materials characterization cell; and (c) means forproviding a controlled flow of oil through said materialscharacterization cell.
 2. A materials characterization system as recitedin claim 1, wherein said materials characterization cell comprises ahousing; a channel disposed in said housing, said channel permitting thepassage of oil through said housing; a first sample of insulationmaterial disposed in said housing adjacent to said channel; and a firstelectrode disposed in said housing adjacent to said first sample.
 3. Amaterials characterization system as recited in claim 2, wherein saidmaterials characterization cell further comprises a second sample ofsaid insulation material, said second sample being disposed in saidhousing adjacent to said channel and opposite said first sample; and asecond electrode disposed in said housing adjacent to said secondsample.
 4. A method for obtaining a polarization spectrum of a sample ofinsulation material for use in an oil filled transformer, saidpolarization spectrum being indicative of the age and moisture contentof said sample, comprising the steps of:(a) disposing said sample in amaterials characterization cell; (b) coupling a polarization spectrumanalyzer to said materials characterization cell; and (c) obtaining saidspectrum with said analyzer.
 5. A method as recited in claim 4, whereinsaid materials characterization cell comprises a housing; a channeldisposed in said housing, said channel permitting the passage of oilthrough said housing; said sample of insulation material disposed insaid housing adjacent to said channel; and a first electrode disposed insaid housing adjacent to said sample.
 6. A method as recited in claim 5,wherein said materials characterization cell further comprises a secondsample of said insulation material, said second sample being disposed insaid housing adjacent to said channel; and a second electrode disposedin said housing adjacent to said second sample.
 7. A method fordetermining the charging tendency of a combination of liquid and solidinsulation materials, comprising the steps of:(a) disposing a sample ofa solid insulation material in a materials characterization cell; (b)passing a dielectric liquid through said materials characterizationcell; (c) measuring a streaming current associated with said dielectricliquid with a meter coupled to said materials characterization cell; and(d) determining the charging tendency on the basis of the flow rate ofthe liquid dielectric and the measured streaming current.
 8. A method asrecited in claim 7, wherein said materials characterization cellcomprises a housing; a channel disposed in said housing, said channelpermitting the passage of said liquid dielectric through said housing;said sample of insulation material disposed in said housing adjacent tosaid channel; and a first electrode disposed in said housing adjacent tosaid sample.
 9. A method as recited in claim 8, wherein said materialscharacterization cell further comprises a second sample of saidinsulation material, said second sample being disposed in said housingadjacent to said channel; and a second electrode disposed in saidhousing adjacent to said second sample.